1. Field of the Invention
The formation of sulfonate containing polymers has been clearly defined in a number of U.S. Pat. Nos.: 3,642,728; 3,836,511; 3,847,854; 3,870,841; and 3,877,530. These patents teach the formation of sulfonate polymers by contacting a polymer having olefinic unsaturation with a sulfonating agent.
This application differs from the previously identified U.S. Patents in that the instant process is directed to the free radical emulsion polymerization of at least one conjugated diene with a sulfonate containing monomer to form a substantially gel free co- or terpolymer having a major portion of a conjugated diene and a minor portion of a sulfonate containing monomer, wherein the co- or terpolymer is water insoluble.
2. Prior Art
Copolymers and terpolymers containing sulfonate monomers at low levels have been described previously in the art. For example, U.S. Pat. No. 3,306,871 describes the preparation of latices based on several different vinyl monomers. This prior art is particularly relevant to the instant invention because it distinguishes those features which are characteristic of much of the prior art in the area of metal sulfonate ionomers.
The maximum wt. % of sulfonated monomer in the formed polymers of U.S. Pat. No. 3,306,871 is 3 wt. % and these polymers are incapable of forming ionically cross-linked polymers with excellent physical properties such as tensile strength, whereas the minimum wt. % of sulfonate monomer in the instant polymers is 4 wt. % (which is equivalent to about 18 meq. of sulfonate groups per 100 grams of polymer) and these polymers are capable of forming highly ionically cross-linked polymers with vastly improved physical properties such as tensile strength.
It has become clear, based on recent work, that sulfonate ionomers can manifest many peculiarities which make their characterization extremely difficult. The foremost of these is a strong ionic cross-linking which makes such materials difficult to dissolve in solution or to achieve melt flow. These techniques are necessary to characterize a polymeric product for subsequent use, for example, as a thermoplastic elastomer or as an oil additive or in similar polymeric applications. The quantification of melt flow properties or selected solution properties therefore becomes an extremely important step in order to know whether these ionomeric products are even being prepared in a reproducible fashion, or whether they are suited for selected applications.
This particular problem is exacerbated even more when describing copolymers of diene monomers and metal or amine sulfonate containing monomers, for in this latter case, the probability of a high degree of covalent cross-linking is extremely likely. When concurrent problems of ionic cross-linking and covalent cross-linking are now combined with problems of uniform copolymers obtained between a relatively non-polar hydrocarbon monomer and a highly polar salt molecule, which are normally completely immiscible, and the normal problems of molecular weight control desired of all polymer systems, it becomes evident why there is a paucity of information concerning the characteristics of the isolated sulfonate ionomers obtained by copolymerization of dienes and metal or amine sulfonate containing monomers. In fact, a review of the patent art and the literature suggests that a complete characterization of such systems has not often, if ever, been previously attempted.
Possibly, for the above reasons, most of the patent art in this area describes the latices achieved by the presumed reaction of such sulfonate monomers with selected vinyl and diene monomers. That the products are copolymers is presumed, by suggestions, that the resulting latices are more stable, more water resistant, and more adherent than those obtained in the absence of the sulfonate monomer.
The instant invention is directed at a different class of copolymers of dienes and metal or amine sulfonate containing monomers. This invention is concerned with the preparation and resultant compositions of dienes and sulfonate monomers which are of desirable molecular weights, are substantially free of covalent cross-linking (less than 10% of the product appears as gel in prescribed tests), contains sulfonate levels of about 18 to about 100 meq. of sulfonate groups per 100 grams of polymer, are water insoluble, and are prepared by a process designed to give products substantially free of homopolymers of any of the polymerizable components, and are solid products, characterizable in terms of reduced viscosity, molecular weight and/or melt flow (at elevated temperatures).
The polymers formed by the instant process are those strictly limited to species wherein the sulfonate groups are affixed only to the aromatic rings. For example, in the copolymerization of sulfonated styrene monomer and butadiene monomer one forms by the instant process, a sulfonated styrene-butadiene polymer, wherein all the sulfonate groups are affixed to the aromatic rings. In contrast, when one sulfonates a polymer containing both aromatic groups and aliphatic groups by a direct sulfonation process such as those disclosed in U.S. Pat. Nos.: 3,072,618; 3,072,619; 4,186,163 and 3,396,136, one obtains a sulfonated species, wherein the majority of sulfonate groups are affixed to the non-aromatic portion of the polymer. The instant experimental results clearly show that when one directly sulfonates a styrene-butadiene copolymer, at least 84 mole % of the sulfonate groups are affixed to the non-aromatic portion of the styrene-butadiene copolymer, wherein, the styrene-butadiene copolymer formed by the instant, unique and novel process has 100 mole % of the sulfonate groups affixed to the aromatic rings of the sytrene portion of the styrene-butadiene copolymer.
In light of the above distinguishing features, it is readily apparent as to why the products and process of the instant invention differ from those of GB No. 895,033. That patent addresses specifically latices based on copolymerization of a suitable aromatic vinyl sulfonic acid derivative with a variety of different polymerizable monomers. It is important to note that all aspects of that invention are solely concerned with the resultant latices, that the products are not descried as isolated entitites.
For the purposes of the instant invention, many of the features of the products in GB No. 895,033 are undesirable. For example, it is emphasized in that application that improved latex stability is an asset, for the purpose of the instant invention that improved latex stability can be a debit in that difficulties may be encountered in isolating the solid polymer.
Another feature of the instant invention is that the instant disclosure is directed toward products which are demonstrated to be substantially free of covalent cross-linking, and techniques whereby the ionic cross-linking desired in such systems can be separated from the covalent cross-linking. This demonstration has not been illustrated in the prior art for copolymers of sulfonate containing monomers with polymerizable conjugated diolefins. Without this illustration it would be difficult, if not impossible, to employ resultant products in some of their intended applications.
Finally, and most importantly, U.S. Pat. No. 3,306,871 and GB No. 895,033, specifically state in column 1, second paragraph of both applications that those inventions were concerned with polymer latices wherein the sulfonic acid salt is incorporated as an anionic stabilizer. It is important to emphasize that in the instant invention, the sulfonate is incorporated at specific levels to function as an ionic cross-linking agent. By that we means that in these systems the neutralized sulfonate provides a salt species which interacts with other salt species to provide a strong ionic cross-link. This characteristic is observed and desired, not in the latex form, but is extremely important in determining the bulk physical properties.
In light of the above discussions, it is apparent that the products of this invention differ markedly from those of the prior art in properties, in composition and in their specific structural features.
In addition, the catalysts or initiators employed, for example, in GB No. 895,033 differ from those employed in the process of the instant invention. The prior art has suggested that either water soluble free radical-generating catalysts or oil soluble, free radical-generating catalysts are suitable. Alternatively, U.S. Pat. No. 2,913,429 specifies that "it is necessary that the polymerization mixture include a water soluble peroxy compound as a catalyst and it is desirable, but not essential, that it also include an oil soluble catalyst ingredient" (column 2, line 28 forward).
The initiators of the instant invention are important and different from those of the prior art for it is believed that they function to give an optimum copolymerization without significant incorporation of either a homopolymer of the hydrocarbon molecule or a homopolymer of the metal sulfonate monomer. If one employs substantial levels of an oil soluble initiator which can spontaneously polymerize solely in the diene phase, then that species can be polymerized without a corresponding incorporation of the sulfonate species. By appropriate selection of one catalyst component soluble in the polymerizable hydrocarbon phase and a second catalyst component soluble in the aqueous phase, then the interaction of these components can result in a more uniform copolymerization, predominantly at the interface.
Another patent which can be considered to be relevant to the instant invention and which teaches ionic cross-linking is that of Rees, U.S. Pat. No. 3,322,734.
U.S. Pat. No. 3,322,734 teaches that ionically cross-linked copolymers may be prepared via direct copolymerization and teaches how such ionic cross-linking can change the properties of polymers. However, that patent specifically is directed at neutralization levels of between 10 and 90% of the acid species present. The instant invention is directed at neutralization levels of 95% and above, and preferably at inomers which are 100% neutralized. The properties of the resulting material, which are 100% neutralized, are substantially different from those which are only 50% or 90% neutralized. Therefore, the instant invention is substantially different from that of U.S. Pat. No. 3,322,734.
In addition to the above art, mention should be made of U.S. Pat. No. 2,913,429 which is concerned with synthetic latices designed to form films which are based on aqueous dispersions of copolymers of one or more aliphatic conjugated diolefins with at least two monovinyl aromatic compounds including a monovinyl aromatic sulfonate.
This invention differs from that prior art in the following:
(1) This application is not concerned with films from latices. PA2 (2) That invention contains from 4 to 35, preferably 5 to 15 wt. % sulfur monomer and thereby provides coatings or films which can readily be removed from substrates by washing or scrubbing with water. Obviously, those cited systems were designed to be water sensitive and thereby removable. PA2 (3) That cited patent requires 93 to 25, preferably from 77 to 45% vinyl aromatic compound such as sytrene. The instant application does not permit more than 10% styrene. PA2 (4) That invention requires a water soluble peroxy compound as initiator, whereas this application requires a hydrocarbon soluble peroxy initiator. PA2 (5) These polymers of U.S. Pat. No. 2,913,429 contain a maximum of 40% wt. % of conjugated diene and are thermoplastic in nature, whereas the elastomeric polymers of the instant invention contain a minimum of 55% wt. % of conjugated diene. PA2 (6) The polymers of U.S. Pat. No. 2,913,429 are water sensitive and films formed from these polymers can be removed from a substrate by washing with water, whereas the polymers of the instant application are water insensitive.
There are many other distinctions, but it is obvious that the above invention is directed at essentially rigid removable paint films, wherein the sulfonate groups provide adequate water sensitivity to permit the formation of a stable latex which further can be deposited as a removable film. Nowhere in that invention is the concept of a metal sulfonate copolymer possessing strong ionic cross-links in the bulk product taught, inferred, or even desired.